Optical transmission employs electromagnetic waves from a spectrum of wavelengths including but extending well beyond visible light, and expressions herein such as "optical", "light" and related terms are accordingly to be understood in the wider sense of referring to electromagnetic waves within this broader spectrum of wavelengths.
In optical communication systems light, modulated in accordance with information to be conveyed, is transmitted along dielectric waveguides.
The majority of optical communication systems presently in operation, of which optical telecommunication systems are an important example, employ a combination of transmission of non-coherent light and direct intensity modulation for conveying digital information.
Considerable advantages in respect of, among others, bandwidth utilization, transmission bandwidths, choice of appropriate modulation techniques, and receiver sensitivity, are envisaged to derive from using coherent light for transmission. Unlike optical communication systems using non-coherent light for transmission, systems using coherent light (referred to hereinafter also as "coherent systems") have to employ narrow line width light sources and, particularly for long distance communication, will generally use low loss, single mode optical fibres as the dielectric optical waveguides.
It has been appreciated for some time now that if narrow line width light, for example from a laser light source, is launched into an optical fibre, and especially into a low loss optical fibre, then there is a threshold power (the natural Stimulated Brillouin Scattering threshold for the fibre at that linewidth) above which Stimulated Brillouin Scattering (hereinafter also referred to as SBS) occurs in the fibre (see for example, R. G. Smith, "Optical Power Handling Capacity of Low Loss Optical Fibres as Determined by Stimulated Raman and Brillouin Scattering", Appl Opt, 1972, II, pp 2489-2494; E. P. Ippen and R. H. Stolen "Stimulated Brillouin Scattering in Optical Fibres", Appl Phys Lett, Vol 21, No 11, Dec. 1, 1972; "Optical Fibre Telecommunications", 1979, Academic Press, New York (US), ed S. E. Miller et al, Chapter 5 "Non Linear Properties of Optical Fibres", pp 125-150, para 5.3; P. Labudde et al, "Transmission of Narrow Band High Power Laser Radiation Through Optical Fibres", Optics Communications, Vol 32, No 3, March 1980, pp 385-390; N. Uesugi et et al, "Maximum Single Frequency Input Power in a Long Optical Fibre Determined by Stimulated Brillouin Scattering", Electronics Letters, May 28, 1981, Vol 17, No 11).
As explained in these references, Stimulated Brillouin Scattering is a stimulated scattering process which converts a forward travelling optical wave into a backward travelling optical wave which is also shifted in frequency. At launched light powers exceeding the above mentioned threshold power, the amount of scattering rises steeply until the power transmitted forward through the fibre becomes nearly independent of the launched power. In addition to thus attenuating the transmitted power, SBS has further detrimental effects such as causing multiple frequency shifts, increased backward coupling into the laser light source and, for sufficiently high launched powers, even permanent physical damage to the fibre.
It should be noted that, although of great importance for coherent systems for which the use of narrow line width sources is mandatory, SBS is of course not confined to coherent systems. Rather, SBS may occur whenever the appropriate conditions are satisfied in respect of line width, launch power, characteristics of the optical waveguide, and so forth.
SBS is only one of several non-linear processes which may occur in optical waveguides, and is generally less significant with broad line width than with narrow line width light. Nevertheless, in view of its threshold being usually lower than thresholds for other non-linear processes, SBS has been considered to present a major limitation for optical communication systems (see the cited references, and in particlar R. G. Smith, P. Labudde, and N. Uesugi). This limitation, which manifests itself by constraints on the maximum practicable launch power, has special significance for coherent systems where, as has been indicated before, there is no option of using broad line width light. A constraint on the launch power has of course a direct effect on the maximum transmission length which can be achieved without recourse to repeaters or regenerators.
The majority of the cited references discusses the aforesaid constraint on the launch power to levels not much greater, and preferably less than the SBS threshold, but in none of the references is it suggested whether and, if so, how this limitation may be overcome. Thus, for example, N. Uesugi et al, (cited above) demonstrate that in the near infrared region SBS will occur in long single mode silica fibres with input powers as low as a few milliwatts. Yet, in spite of their investigations having been carried out in view of the importance of SBS for coherent communication systems, the authors fail to suggest even the existence of a suitable remedy.